Emotions can spread among users of online social networks such as Facebook, a new study has found.

Social scientists at Cornell University, the University of California, San Francisco (UCSF), and Facebook, reduced the amount of either positive or negative stories that appeared in the news feed of 689,003 randomly selected Facebook users, and found that the so-called "emotional contagion" effect worked both ways.

"People who had positive content experimentally reduced on their Facebook news feed, for one week, used more negative words in their status updates," said Jeff Hancock, professor of communication at Cornell's College of Agriculture and Life Sciences and co-director of its Social Media Lab.

The experiment is the first to suggest that emotions expressed via online social networks influence the moods of others, the researchers said.

Researchers in the study never saw the content of actual posts, per Facebook's data use policy; instead, they counted only the occurrence of positive and negative words in more than 3 million posts with a total of 122 million words.

They found that 4 million of those words were "positive" and 1.8 million were "negative."Hancock said peoples' emotional expressions on Facebook predicted their friends' emotional expressions, even days later.

"We also observed a withdrawal effect: People who were exposed to fewer emotional posts in their news feed were less expressive overall on the following days," Hancock said.

"This observation, and the fact that people were more emotionally positive in response to positive emotion updates from their friends, stands in contrast to theories that suggest viewing positive posts by friends on Facebook may somehow affect us negatively," he added.

"In fact, this is the result when people are exposed to less positive content, rather than more," he said.